The present invention relates generally to electrostatic discharge (ESD) in integrated circuits, and, more particularly, to an apparatus and method for improved triggering and leakage current control of electrostatic discharge (ESD) power clamping devices.
Electrostatic Discharge (ESD) events, which can occur both during and after manufacturing of an integrated circuit (IC), can cause substantial damage to the IC. ESD events have become particularly troublesome for complementary metal oxide semiconductor (CMOS) chips because of their low power requirements and extreme sensitivity. A significant factor contributing to this sensitivity to ESD is that the transistors of the circuits are formed from small regions of N-type materials, P-type materials, and thin gate oxides. When a transistor is exposed to an ESD event, the charge applied may cause an extremely high current flow to occur within the device, which in turn can cause permanent damage to the junctions, neighboring gate oxides, interconnects and/or other physical structures.
In particular, there are three general types of ESD events that have been modeled: the human body model (HBM), the machine model (MM) and the charged device model (CDM). The HBM and MM represent discharge current between any two pins on an IC as a result of (respectively) a human body discharging through a chip and a metal tool discharging through a chip. Whereas a human body discharge is relatively slow in terms of rise time and has a unidirectional current in the range of about 1-3 amps, a tool discharge is a relatively rapid event that produces an even higher, bi-directional current into and out of the pin (e.g., about 3-5 amps). In the CDM, the ESD event does not originate from outside the IC device itself, but instead represents the discharge of an IC device to ground. The IC device is charged through the triboelectric effect (frictional charging) or by an external field. The charging of the device substrate itself does not subject the IC to ESD damage, but rather the discharging. As is the case with the MM, the CDM is a very rapid event.
Because of the potential damage from such types of ESD events, on-chip ESD protection circuits for CMOS chips have become commonplace. In general, such protection circuits include ESD clamps configured to maintain the voltage at a power line to a value that is known to be safe for the operating circuits, and that will also not interfere with the operating circuits under normal operating conditions. An ESD clamp circuit is typically constructed between a positive power supply (e.g., VDD) and a ground plane, or a ground plane and a negative power supply (e.g., VSS). The primary purpose of the ESD clamp is to reduce the impedance between the rails VDD and VSS so as to reduce the impedance between the input pad and the VSS rail (i.e., discharge of current between the input to VSS), and to protect the power rails themselves from ESD events.
The continued scaling of semiconductor devices has resulted in some unique challenges for providing ESD protection in CMOS applications. For example, ESD power clamps should ideally provide low power off states by minimizing leakage current dissipated therethrough in order to minimize battery power loss, but at the same time should still maintain a desired level of ESD protection. Unfortunately, existing circuit topologies that provide sufficient ESD protection do not also minimize the leakage state, since the capacitances associated with providing the ESD time coverage are also leaky, and since the trigger device has a relatively large time constant associated therewith. Moreover, these larger capacitors consume relatively large amounts of device real estate. Accordingly, it would be desirable to be able to provide customers with sufficient ESD protection while also minimizing leakage losses, particularly for applications where power consumption is of concern.